Allowable Stress of I-beam

Such a beam installed on a spacecraft might have a safety factor of only 1.2....

You are comparing two separate design cases. For the Service Load case you use a proportion of yield stress, for Ultimate Load, the loads are factored up and compared with a proportion of Ultimate Stress.

The problem is that depending on the "shape" of the beam, it might be legal to use 0.66Fy, it depends on whether the shape is Compact according to the code you should be using. This may help

Allowable Stress Design and Load and Resistance Factor Design.

The Australian Standards specify a maximum stress of 60% on the Yield.

From ASME, Section II, Part D, the max. allowable stress for material ASME SA-36 (Bar, Plate & Sheet) is 114 MPa at temp. and up to 350 ^oC, and its min. yield strength is 250 MPa.

And if we use bending strength as o.6 of Min. Yield Strength = 0.6 x 250 = 150 Mpa.

There's several reasons why you will not get a satisfactory & "down and dirty" answer in this forum.

First, you're using SI units in here when most of the structural engineers from the USA and CA in here are using English Empirical units.

Second, we (and you) need to know a lot of conditions concerning the design of the steel beam or column or plate (or whatever it is you're designing), as well as loads, lateral support, span or heights, etc etc. Also, what type of steel shape is it that you're using. All of these condition factors have a direct bearing and correlation on determining the allowable stress. You also need to follow your country's national building code, as it may stipulate what structural engineering and load codes you must apply to your analysis and design, whether it be BOCA, ASCE Standard 7, or whatever. Also, you need to choose whether you use the AISC ASD or LFRD design philosophy; normally your national building code will dictate which one to employ.

I think you had better obtain an ebook copy of the latest AISC manual and start studying it from cover to cover. If yo normally don't do structural steel design then you have no business doing so, as you'll end up killing someone and creating a pile of collapsed building...do you want to go to prison for your bad design and actions?

Lastly, I strongly suggest that you hire a licensed and registered structural engineer to undertake this design. You apparently do not know what you re doing in terms of proper structural steel design!

The First methode is Allowable Strength design (ADS) theroy the second is Load Resistance Design Factor (LRFD). They are both right you need to know what you are doing.

Apart from what others have posted, you also need to take account of deflection. It's quite common for section size to be limited by deflection rather than stress. If so the stress could be lower than both your figures, and there's no decision to take.

Codey